Question

This is the code :

import java.awt.*;
import javax.swing.*;

class tester {
JFrame fr;
JPanel p;
Graphics g;

tester() {
 buildGUI();
}

public void buildGUI() {
 fr=new JFrame();
 p=new JPanel();
 p.setBackground(Color.red);
 g.setColor(Color.black);
 g.drawOval(18,45,78,39);
 g.fillOval(18,45,78,39);
 fr.add(p);
 fr.setVisible(true);
 fr.setSize(500,500);
}

 public static void main(String args[]) {
  new tester();
 }
}

These are the exceptions produced when i try to run the code :

Exception in thread "main" java.lang.NullPointerException
at tester.buildGUI(tester.java:17)
at tester.<init>(tester.java:10)
at tester.main(tester.java:26)


Why do i get these exceptions?
How can i solve it.

Answer 1

You never created object g – you just declared it.

Until you create an object and assign it to the variable holding a reference to it, the value of that variable is null.

That’s why you are getting NullPointerException here.

For example:

//created a variable holding a reference to an object of type JPanel
JPanel p;
//now the value of p is null. It's not pointing to anything

//created an object of type JPanel and assigned it to p
p=new JPanel();
//now p is not null anymore, it's pointing to an instance of JPanel

Well, you didn’t do that for Graphic object g.